Friday 3 June 2011 13.53 EDT
First published on Friday 3 June 2011 13.53 EDT

Three male painting contractors and a female fire-watch officer are believed to have died in the huge explosion at the Chevron refinery at Pembroke Dock, south-west Wales, it emerged as police and safety experts launched investigations.

Health and Safety Executive staff sealed off smoke-blackened sections of the extensive complex where a large storage tank blew up late on Thursday during routine maintenance, rocking houses and sending a plume of black smoke into the sky. A fifth worker was critically ill with severe burns.

The complex, one of the largest in western Europe, has a good safety record but is at constant risk of serious accidents.

The four who died have not been formally identified. The injured worker is being treated at the Morriston hospital in Swansea, where he was airlifted by the Wales air ambulance service.

Chevron said that the refinery was fully operational but only essential work was being undertaken as a mark of respect. The general manager, Greg Hanggi, said support was being arranged for all of the 1,400 staff who needed it.

A spokesman for Chevron, which is carrying out its own inquiry, said: "Tank maintenance is regularly carried out and this was planned work."

The Welsh first minister, Carwyn Jones, sent a message of sympathy and the Welsh secretary, Cheryl Gillan, said: "We have asked to be kept fully informed about the investigation."

The refinery dominates the town and underpins the local economy. Accidents in the dangerous processes involving volatile fuels and gases have been part of the area's history in spite of all precautions.

The last serious one was in 1994, when an explosion and fire damaged property in Milford Haven, seven miles away.

Tony Spicer, 75, a retired welder at Chevron who lives close to the plant, said: "At a refinery you are working within a time-bomb and at any time something can go wrong. I think that an accident and loss of life is inevitable from time to time. It's not always human error, it is just something that can happen.

"If the refinery finished tomorrow, a lot of people would be out of work and it would hit the local economy. Everyone from the milkman to shopkeepers would be affected."

Huw Morgan, 83, whose cottage at Newton is a mile from the refinery, saw the explosion from his garden. He said: "There was an enormous black cloud of smoke going up like a mushroom."

Work on the nearby power station being built for npower has also stopped as a mark of respect.

Mid-Wales fire and rescue service said there had been no health risk from fumes and debris flung out by the explosion. Assistant chief fire officer Chris Davies said: "The wind was blowing offshore, away from residential areas."